1. Field of the Invention
The present invention generally relates to an electro-optical reader for reading indicia such as a bar code symbol and, more particularly, to generating a full coverage, omni-directional scan pattern at a vertical window of the reader to insure reliable reading of the symbol through the window.
2. Description of the Related Art
Flat bed laser readers, also known as horizontal slot scanners, have been used to electro-optically read bar code symbols, particularly of the Universal Product Code (UPC) type, at a point-of-transaction workstation in supermarkets, warehouse clubs, department stores, and other kinds of retailers for many years. As exemplified by U.S. Pat. Nos. 5,059,779; 5,124,539 and 5,200,599, a single, horizontal window is set flush with, and built into, a horizontal countertop of the workstation. Products to be purchased bear an identifying symbol and are slid across the horizontal window through which a multitude of scan lines is projected in a generally upwards direction. When at least one of the scan lines sweeps over a symbol associated with a product, the symbol is processed and read.
Instead of, or in addition to, a horizontal slot scanner, it is known to provide a vertical slot scanner which is typically a portable reader placed on the countertop such that its window is generally vertical and faces an operator at the workstation. A scan pattern generator within the vertical reader projects a multitude of scan lines in a generally outward direction through the vertical window toward the operator. The operator slides the products past the vertical window from right to left, or from left to right, or the operator merely presents the symbol on the product to the center of the vertical window, the choice depending on operator preference or on the layout of the workstation.
Each product must be oriented by the operator with the symbol facing away from the operator and directly towards the vertical window. Hence, the operator cannot see exactly where the symbol is during scanning. In typical “blind-aiming” usage, the operator first visually locates the symbol on the product, and then turns the product about a vertical axis to aim the symbol at the reader. It is not uncommon for the operator to unintentionally position the symbol in a plane rotated about 20 to 30 degrees away from the plane of the window.
The blind-aiming of the symbol is made more difficult because the position and orientation of the symbol are variable. The symbol may be located low or high, or right to left, on the product, or anywhere in between. The symbol may be oriented in a “picket fence” orientation in which the elongated parallel bars of the one-dimensional UPC symbol are vertical, or in a “ladder” orientation in which the symbol bars are horizontal, or at any orientation angle in between.
In such an environment, it is important that the scan lines located at, and projected from, the vertical window provide a full coverage scan pattern which extends down as close as possible to the countertop, and as high as possible above the countertop. The scan pattern must include scan lines oriented to read symbols positioned in any possible way across the entire height of the scan pattern.
In some installations of a vertical slot scanner, there is insufficient available work space to slide products across the countertop in front of the vertical window, in which case, the operator will typically present the symbol by holding the product more or less stationary in front of the vertical window. Many operators intuitively expect the reader to work best when the symbol is presented to the center of the vertical window. Yet, the known vertical slot scanners have “dead zones” empty of scan lines at upper central regions of the vertical windows and thus cannot read symbols in the picket fence orientation when the symbols are located above the central regions of the windows. Some of the known vertical slot scanners also have such dead zones at lower central regions of the vertical windows and thus cannot read symbols in the picket fence orientation when the symbols are located below the center regions of the windows. These dead zones result in some products not being read the first time that they are slid past, or presented to, the window, thereby slowing down transaction processing and reducing productivity.
All known vertical slot scanners have scan patterns that grow as they are projected into space in front of the window. This allows them to cover a larger area on products positioned several inches away from the window and, indeed, reader performance improves if the product is positioned more remote from the window. However, many operators intuitively feel that the reader will work best if the symbol is positioned right up close to the window. Hence, to deal with real life usage, it is important that the scan pattern fills the entire height of the window, and then grows rapidly as the scan pattern is projected into space in front of the window.
A known scan pattern generator in a vertical slot scanner in accordance with the prior art includes a laser for emitting a laser beam at a mirrored component mounted on a shaft for rotation by a motor about an axis. A plurality of stationary mirrors is arranged about the axis. As the mirrored component turns, the laser beam is successively reflected onto the stationary mirrors for reflection therefrom through the vertical window as a scan pattern of scan lines. However, as noted above, the scan pattern of the prior art vertical slot scanner does not entirely fill the vertical window and has dead zones, especially above and below the central region of the vertical window, which limit reader performance.